1. Field of the Invention
The present invention relates to a method of producing a molded article, an extruder, and a molded article. Particularly the present invention relates to a method of producing a molded article using a high-pressure fluid, an extruder used in this production method, and a molded article which is produced by this production method.
2. Description of the Related Art
As a forming method (production method) for creating a molded article with thermoplastic resin (referred to as “resin molded article” hereinafter), injection molding and extrusion molding have been widely used conventionally. In the extrusion molding, a thermoplastic resin material such as pellet is plasticized and molten by a screw rotating inside a heating cylinder of an extruder and the molten resin is continuously poured into an internal pressure die (base), whereby a molded article with a constant cross sectional shape can be obtained. Therefore, extrusion molding is suitable as a method for continuously producing, for example, a thermoplastic resin film, sheet, pipe, and the like.
There has been conventionally proposed a method in which supercritical fluid is used for the above-mentioned extrusion molding. Specifically, there is proposed a foam extrusion molding method for forming fine foam cells inside a molded article by using supercritical fluid as a pneumatogen (see, for example, Japanese Patent Application Laid-Open No. 2001-341186). Moreover, there is proposed an extrusion molding method of mixing and milling supercritical fluid with molten resin while improving plasticization/melting performance of a material which has high glass-transition temperature and is difficult to be molded by using supercritical fluid as a plasticizer (see, for example, Japanese Patent Application Laid-Open No. 2002-273777).
Furthermore, extrusion molding is expected as a method of molding a liquid crystal polymer film and a method of forming a metallic thin film on a surface of the polymer film. The reason is because liquid crystal polymer, which is thermoplastic resin, has high heat resistance and excellent thermal dimensional stability and a high-frequency electric property, and thus is promising as a material such as a flexible circuit board.
However, generally, because thermoplastic liquid crystal polymer has low melting viscosity, and molten liquid crystal polymer molecules are easily oriented in one direction when being extruded from a die, thermoplastic liquid crystal polymer has a disadvantage that a liquid crystal polymer film is split easily in one direction. Therefore, in order to eliminate such disadvantage of liquid crystal polymer film, there is proposed a method of molding a liquid crystal polymer film by means of inflation molding (see Japanese Patent Application Laid-Open No. H2-88212). In Japanese Patent Application Laid-Open No. H2-88212, there is proposed a method of braking one direction of the molecular orientations in an unsolidified (semisolid) film obtained by being ejected from a die, the liquid crystal polymer molecules in the film being oriented in one direction. Here, inflation molding is a method in which molten resin is extruded through a circular die (spiral die or the like) to mold a tubular resin bubble, then air is blown into the tubular resin bubble to expand and cool the resin bubble, and thereby a tubular film having a desired diameter is molded.
Moreover, as a method of forming a metallic film on a resin molded article, a spattering method, a vapor deposition method, or the like are conventionally known. However, costs are high for these methods since the devices used are expansive and vacuum processing is performed. On the other hand, as a low-cost metallic film formation method, an electroless plating process is commonly known.
The procedures for forming a metallic film on a resin molded article by the electroless plating process vary slightly in accordance with the materials to be used. However, normally, after a resin molded article is produced in a molding process such as an extrusion molding process, the molded article is degreased. Next, various steps of etching, neutralization and wetting, addition of catalysts, and activation of the catalysts are carried out, and thereafter the molded article is immersed in electroless plating solution to form a metallic film on the molded article. In electroless plating, a film having high reflectance, such as a gold film, silver film, copper film, or nickel film can be coated as the metallic film on a polymer surface. Moreover, there is a case in which the molded article on which an electroless plating film (metallic film) is formed is subjected to electrolytic plating to make the thickness of the metallic film thicker.
The etching step in the above electroless plating process is performed for the purpose of improving the adhesion between the catalysts and the resin due to an anchor effect obtained by roughening a surface of the molded article. In the etching step, chromic acid solution, alkali metal hydroxide solution or the like is used as the etching solution, but such etching solution is required to be subjected to after-treatment such as neutralization and the like, thus use of the etching solution is a factor of increased cost. Moreover, since such etching solution has high toxicity, the problem is that the handling of such etching solution is troublesome.
Therefore, there have been conventionally proposed several plating processes which do not require roughening of a surface of the molded article by etching. For example, there is proposed a process of forming a thin film, which contains a plating catalyst, on a plastic surface by using an organic binder or ultraviolet curable resin (see, for example, Japanese Patent Application Laid-Open No. H9-59778 and Japanese Patent Application Laid-Open No. 2001-303255). Further, there is proposed a technology in which an ultraviolet laser is irradiated onto a plastic surface in an atmosphere of gas such as amine compound to modify the plastic surface (see, for example, Japanese Patent Application Laid-Open No. H6-87964). There is also proposed a technology for modifying a plastic surface by means of corona discharge processing, plasma processing, ultraviolet treatment or the like.
Moreover, in order to overcome the problem of the above-mentioned conventional technology of forming electroless plating film, there is proposed an electroless plating method for plastic using supercritical fluid (see, for example, “Latest Applied Technology of Supercritical Fluid”, Teruo Hori, NTS Inc. Pg. 250 through 255, 2004). According to the method described in “Latest Applied Technology of Supercritical Fluid”, organic metal complexes are dissolved in a supercritical carbon dioxide, thus the supercritical carbon dioxide is brought into contact with various polymers, and thereby metal complexes are injected into polymer surfaces. Then, the metal complexes are reduced by applying heat thereto or by means of chemical reduction treatment to deposit the metallic fine particles on the polymer surfaces. Accordingly, the entire polymer surfaces can be subjected to electroless plating. Therefore, it is mentioned in “Latest Applied Technology of Supercritical Fluid” that according to this process, waste liquid treatment is not required to be performed, thus an electroless plating process in which a resin with excellent surface roughness can be obtained is achieved.
Incidentally, as an example of the molded article which is obtained by forming a metallic film on a resin molded article, there is a light reflecting body such as a reflector used in an automotive lamp, and recently instead of using thermoset resin, thermoplastic resin such as polyester resin and polycarbonate resin that can be produced massively is used as a resin material for such light reflecting body. Since the temperature inside the head lamp increases, a reflector which has a heat resistance of 160° C. through 180° C. and in which the metallic reflection coatings are not separated is required.
As a method of forming a metallic reflection coating on the surface of such molded article (polymer base material), there is proposed a method of forming a metallic film by means of vapor deposition (direct vapor deposition method) after the surface of the molded article is subjected to plasma activation. However, in the reflector which is produced in this method, there is a problem that the light reflection coating is fogged when the reflector is held in a high-temperature environment for a long time. The cause of this fogging is separation between the metallic film and the polymer base material. The causes of the separation include thermal expansion or deformation of the polymer base material, thermolysis gas generated from the polymer base material, and the like.
As a method of preventing the occurrence of the thermolysis gas from the polymer base material, for example, a method for improving the resin components is proposed (see, for example, Japanese Patent Application Laid-Open No. 2000-35509). The resin composition described in Japanese Patent Application Laid-Open No. 2000-35509 prevents the occurrence of the pyrolysis gas of the polymer base material by reducing the amount of distal carboxyl group to the specified amount or less. However, the resin composition described in Japanese Patent Application Laid-Open No. 2000-35509 does not contain a mold release agent, thus mold release failure may occur when molding a large, complicated three-dimensional shaped molded article. As a method for improving such problem, a method for improving the mold release agent of resin component is proposed (see, for example, Japanese Patent Application Laid-Open No. 2005-97563).
As a structural body in which a metallic film is formed on a surface of a plastic molded article, there is an fθ mirror which is used for scanning an optical beam of a laser printer or a copying machine, and a large mirror which is used for deflecting an optical path of a projection television. In a method of producing such plastic mirrors, conventionally, a metallic reflection coating is formed by means of vapor deposition or the like on a surface of a molded article to which a high-precision mirror plane of a mold is transferred. Such a vapor deposition method requires an expensive deposition apparatus, thus the plant cost is high. Especially for a large-area molded article, only a small number of molded articles are obtained per one batch at the time of vapor deposition, leading to poor productivity.
In order to overcome the above problems, there is proposed a conventional method of creating a plastic mirror by attaching a metallic sheet to a mold surface with vacuum and simultaneously performing composite injection molding with thermoplastic resin, without carrying out the vapor deposition step (see, for example, Japanese Patent Application Laid-Open No. H5-315829). However, the method proposed in Japanese Patent Application Laid-Open No. H5-315829 has a problem that the reflectance of the metallic film becomes low. Moreover, in the method proposed in Japanese Patent Application Laid-Open No. H5-315829, since the metallic sheet needs to be bonded to the mold, it is difficult to make a tracing of a complicated shape of a mold with the metallic sheet.
Furthermore, there is proposed a method of obtaining high reflectance by using a metallic reflective film such as an aluminum or silver reflection coating on a resin film (transfer sheet) instead of on the metallic sheet (see, for example, Japanese Patent Application Laid-Open No. H3-82513). The method proposed in Japanese Patent Application Laid-Open No. H3-82513 can be performed easily as long as a flat molded article is used, but in the case of a complex molded article with curved surfaces, a tension may be applied to the film and thereby cracks may be caused on the metallic reflective film during the process where the metallic reflection coating is transformed to have curved surfaces when molding is performed. The reason of such occurrences is that the bonding strength of the metallic reflection coating formed on the resin film by means of vapor deposition is weaker than the bonding force of the resin film. Moreover, the method proposed in Japanese Patent Application Laid-Open No. H3-82513 has a problem that the metallic reflection coating is easily broken due to the influence of a flow of the molten resin since injection molding is performed to obtain an integrated molded article. It should be noted that in the method of producing the molded article which is described in Japanese Patent Application Laid-Open No. H3-82513, the resin film (transfer sheet) is detached after molding is performed.
As a method of preventing the occurrence of the cracks on the metallic film, there is proposed a method of adhering a polymer base material on which a metallic film is formed by means of vapor deposition, to a polymer molded article, which is obtained by means of injection molding, by means of pressure bonding (see, for example, Japanese Patent Application Laid-Open No. 2004-148638). In the method disclosed in Japanese Patent Application Laid-Open No. 2004-148638, heating/pressure bonding is required in other step after the injection molding, thus mass productivity may be obstructed.
On the other hand, conventionally, as the method of forming a metallic film on a polymer surface at low cost, the electroless plating method are known as described above, but the polymer surface is required to be roughened by means of etching with chromic acid or the like, thus the applicable resin material is limited to ABS or the like which is immersed in the etching solution. In other material such as polycarbonate, a grade, in which ABS or elastomer is mixed so that electroless plating can be performed, is commercially available, but such material does not sufficiently satisfy the requirements of heat resistance and reflective performance.
Moreover, silver mirror reaction is conventionally known as a method of obtaining a high-reflectance metallic film by means of wet electroless plating method. Various reflectors or mirrors in which a silver thin film is formed on a glass by means of the above-mentioned method are widely used. However, application of such silver mirror reaction to a resin molded article having a high degree of freedom in the shape thereof has not been a trend. Also, as described above, the applied material for etching and the resin material which can fix the catalyst core are limited to ABS or the like, thus it is difficult to apply the silver mirror reaction to the resin material having high heat resistance. As a method of forming a reflection coating on a resin base material by silver mirror reaction, there is proposed a method of activating the surface by means of, for example, plasma processing, corona discharge processing or the like (see, for example, Japanese Patent Application Laid-Open No. 2000-73178). In the method disclosed in Japanese Patent Application Laid-Open No. 2000-73178, a transparent curing film made of acrylic resin or the like is required between the base material surface and the reflection coating, and a molded article with a silver reflection coating having a heat resistance of 150° C. or more is not proposed.
Furthermore, the inventors propose an injection molding method in which carbon dioxide of supercritical fluid or the like is used as a solvent to dissolve metallic fine particles of metal complexes in the carbon dioxide, and the metallic fine particles are impregnated in a mold cavity or the like at the time of injection molding to segregate the catalyst core of electroless plating to the surface (see, for example, Japanese Patent Application Laid-Open No. 2005-205898). According to the investigation performed by the inventors, only the metallic fine particles present in the vicinity of the surface of the molded article contribute as the catalyst core of electroless plating, and the metallic fine particles impregnated an inside of the molded article become the loss. Moreover, in the method disclosed in Japanese Patent Application Laid-Open No. 2005-205898, it is difficult to control the depth of penetration of the metallic fine particles and to impregnate the metallic fine particles evenly in the molded article surface because the surface viscosity of the resin is different at the time of dispersion in the mold.